Part A: c for cost. c=0.31m+0.5
0.31m is the cost per minute. 0.5 is cost per call.
Part B: 0.31m+0.5=5.15 to solve we must rearrange.
subtract 0.5 from each side giving us 0.31m=4.85
divide by 0.31 giving us m=15.65
3x -4 (5 - x) = 7x -20
3x-20+4x=7x-20
7x-20=7x-20
The exponential equation (in thousands) is:

plug in t=3 yrs, and solve for 'r'.

Find value of car when t = 10
The answer is -6.833333333